1. Field of the Invention
This invention relates to rotary pan pelletizers, which are used to convert a powdered or finely divided material, such as a batch of glass-making ingredients, into dust-free pellets.
2. Description of the Prior Art
Known pelletizers of this kind comprise a circular pan with a circumferential rim, mounted for rotation about an inclined axis. A plurality of stationary ploughs and/or oscillating scrapers are usually disposed at different radial distances from the axis and with their lower surfaces immediately above the bottom surface of the pan. The powdered or finely divided material, either dry or pre-wetted, is delivered into the rotating pan and sprayed with a liquid. In the case of a glass-making batch, the liquid may be water and/or an aqueous solution of a binder, such as caustic soda, which may provide a part or the whole of the Na.sub.2 O content of the batch. As the pan rotates, the material forms nuclei which agglomerate and are compacted into pellets as they are rolled around the pan in well-defined streams, until the pellets have achieved the desired size, when they pass out over the rim and are collected for use.
It is normal practice to dispose the ploughs or scrapers so as to permit a bed of the material, e.g. from 5 to 20 mm thick, to build up on the bottom surface of the pan as a result of the impingement of the material and aqueous liquid on the surface. The ploughs or scrapers, apart from helping to define the streams of pellets, scrape the bed smooth so that the pellets can roll and agglomerate on it in the desired controlled manner. The bed also protects the bottom surface of the pan from wear. If the bed builds up to an excessive thickness, however, a considerable amount of the material will be scraped off it by the ploughs or scrapers in the form of lumps. This lumpy material, particularly when scraped off by the plough nearest to the pan axis, can be out of specification for size, composition and strength, as it has not passed through the normal cycle of nucleation, agglomeration and compaction, and can cause instability in the running of the pelletizer.